ZENER datasheet Vz & VR confusion

[HINDI]

Hi

i am trying to select a zener diode with Vz =15--18 v .
but in the datasheet 2 voltages are mentioned for each zener

[1N4748A ]

VZ = 22 and Vr=16.7 "TEST VOLTAGE".

so , according to which one , should i select my zener ?

and what is the diffrence ?
there is a note for Vz [note 4] : voltage measurment sto be performed 90 second after application of the dc .

i cann't get it.
see the datasheet

https://alldatasheet.com/datasheet-pdf/view/MICROSEMI/1N4748A.html

salam
Hindi

 

[VVV]

Vz is what you are looking for.

Vr is the maximum reverse voltage for which no significant current flows through the Zener (actually the current is Ir, in the µA range). Also called maximum withstand voltage. It is useful in circuits where you want to use the Zener to create a threshold, or to clamp voltages.
As the voltage is increased, the current also increases, reaching the value Iz, as the voltage reaches Vz.

For example, the 1N4746A is an 18V Zener. The maximum voltage you can apply to it without causing a significant current to flow through it is 13.7V. At this voltage, a maximum of 5µA will flow. Im many circuits, at this current level the Zener is considered "off". If you were to use this in a clamping circuit, the voltage would be relatively undistorted below this level. In a threshold circuit, this would be close to the threshold. A voltage increase above this level causes more significant current to flow and possibly trigger the circuit.